The invention relates to superconducting magnet coil windings in general and more particularly to an improved cooling arrangement for such a winding.
More specifically the present invention is directed to an improvement in an arrangement for cooling a magnet coil winding containing conductors of superconductive material which are cooled by means of a forced flow of a coolant which is fed into the winding at at least one coolant connecting point and is discharged again from the winding at at least one further coolant connecting point. The conductors are subdivided into conductor zones, the operating points of which, being fixed by the current density I, the field strength H and the temperature T, have different deviations from the transistion point of the superconductive material from the superconducting to the normally conducting state which is closest in an I-H-T space and is determined by the critical current density I.sub.c, the critical field strength H.sub.c and the critical temperature T.sub.c.
Magnet windings with superconductors can be used to advantage for producing strong magnet fields with large physical dimensions. Conductor materials which can be considered for this purpose are, for instance, niobium-zirconium or niobium-titanium alloys as well as niobium-tin compounds. Conductors of these superconductor materials are generally stabilized by a normally conducting material and are, for instance, embedded in a matrix of this material. With this measure, the destruction of the superconductors in the event of an uncontrolled transistion of its parts consisting of the superconductor material from the superconducting to the normally conducting state is to be prevented. In arrangements for cooling superconducting magnets of great dimensions "forced" cooling is often provided (cf., CERN-Report 68-17, Nuclear Physics Division, Geneva, May 13, 1968). With this cooling technique, a coolant, for instance, liquid helium, is continuously pumped through discrete cooling canals which are developed in the winding. Cooling canals can be formed, particularly, by appropriate voids in the superconducting conductors themselves. Such conductors are therefore generally called hollow conductors. With this cooling technique, a helium bath cryostat, which would otherwise be required for cooling the winding of the magnet coil, can be dispensed with and can be replaced by a simple vacuum chamber which encloses the winding and merely serves for the thermal insulation of the winding from the outside. In a magnet winding with hollow conductors or corresponding cooling canals arranged between adjacent conductors, the amount of liquid coolant required for cooling the winding can be reduced considerably over a magnet of approximately equal size with coolant bath cooling. This is of advantage particularly in the event of a transistion of the winding from the superconducting to the normally conducting state, because then only relatively little liquid coolant can evaporate. In addition, magnet windings with hollow conductors, contrary to most windings with a cooling bath, can have any orientation in space. Changes in position during operation are then also possible.
The operating data for the conductors of such a magnet coil winding are different within the winding during undisturbed operation. This means that the winding has conductor zones, the operating data of which with respect to the superconduction properties are more critical than the data of adjacent conductor zones. The operating point of such a critical conductor zone defined by the operating data is therefore closer to the nearest transition point from the superconducting to the normally conducting state, determined by the critical data of the superconductive material of the conductors, than the operating points of other conductor zones. This transition point is determined mainly by the critical current density I.sub.c, the critical field strength H.sub.c or the critical magnet induction B.sub.c, respectively, and the critical temperature T.sub.c of the conductor material and is located on a three-dimensional surface in the I-H-T space for which the superconducting state is present, from those for which only normal conduction prevails (Proc. IEE, IEE Review, vol 199, no. 8R, Aug. 1972, page 1007). If, for instance, a conductor zone is situated in a zone of particularly high magnetic field strength which is greater than the field strength in adjacent conductor zones, then the operating data of this conductor zone are closer to the associated transition point than in adjacent conductor zones if the temperature and current density conditions in the conductor zones being compared to each other are at least approximately equal.
An unintentional transition of a superconducting magnet winding to the normally conducting state, which is also called a "quench", often starts from such a critical conductor zone of the winding which is subject to particularly extreme conditions, for instance, particularly high magnetic field strength or a particularly large heat exposure. In order to prevent, in the event of such a quench, the normal conducting zone from spreading through heat condition relatively fast over the entire coil and therefore, accordingly, a situation where much energy must be taken out of the magnet, one will generally endeavor to obtain particularly good cooling of these critical zones. Heretofore, it has been attempted to ensure this by feeding the coolant into the magnet at least in the vicinity of these critical zones, since it is still coolest there and can therefore remove more heat. However, if the winding becomes normally conducting in this critical conductor zone, for instance, because of the particularly high field strength prevailing there, the increased temperature generated by the flow of the electric current is not only passed on lengthwise and crosswise to the conductors in adjacent conductor zones due to heat conduction, but is transported into these conductor zones also by the heated coolant.
The present invention is thus based on the insight that a problem exists in the known arrangements for forced cooling of superconducting magnet windings in that these arrangements further aid the spreading of the normally conducting zone through the coolant. It is therefore an object of the present invention to provide an arrangement for cooling a superconducting magnet winding in which this danger does not exist.